Diamond core drilling utilises an annular drill bit connected to a core barrel assembly. The core barrel is attached to the end of a number of tubular drill rods connected to form a drill string. The drilling progressively removes cylindrical cores of rock or material through which the drill and drill tube advance using a sequence of runs. This type of drilling utilises an inner tube assembly which has an inner tube connected to a head assembly to receive the core sample. The head assembly comprises a latch body connected to a valve housing which in turn is connected to a bearing housing which in turn is connected to an inner tube connector. The inner tube assembly connects to the inner tube connector and may comprise an inner tube, core lifter and core lift case. The inner tube assembly locates within a core barrel which comprises a combination of drill bit, reamer, outer tube, landing ring and locking coupling. The inner tube assembly can be retrieved from the surface when the inner tube is full. Empty inner tube assemblies can be delivered from the surface to the bottom of the drill string in order to recommence drilling.
The drill bit is advanced by rotating the drill string while applying downward pressure. In addition, drilling fluid such as water or drilling muds are pumped through the centre of the drill string, past the inner tube assembly and through the end of the drill bit in order to carry cuttings and other drilling debris to the surface via the annulus between the wall of the hole and the external surface of the drill string.
The hole being drilled may range from vertical, angled downwardly, horizontal, inclined upwardly or directly upwardly. The holes being drilled may be either normal or dry. In dry holes, the drilling fluid drains away or partially drains away naturally through crevasses or other openings in the rock strata through which the drill passes. In normal holes, the drilling fluid does not drain away. Hereafter, normal holes, which retain water or partially retain water will be referred to as wet holes.
Of course, in the case of horizontal or near horizontal holes, it is likely that drilling fluid would naturally drain away particularly when the inner tube assembly is being retrieved or after the inner tube assembly is pumped back into the end of the drill string.
One practice in inserting an empty inner tube assembly from the surface back into the drilling end of the drill string, to use fluid pumped in behind the inner tube assembly to push it along the drill tube into the core barrel and into its drilling position. An empty inner tube assembly is placed into the drill tube at the surface and a water swivel is screwed into the end of the drill tube which allows for fluid to be pumped into the drill tube. This pushes the inner tube assembly to the end of the drill tube where it latches in place in its drilling position in the core barrel prior to recommencing the drilling operation.
Once the inner tube assembly is latched in place, an increase in pressure of the fluid being pumped will result and is sensed and provides the indication that the inner tube assembly has been pumped into its drilling position and is latched. In order to achieve this and in order to allow the inner tube assembly to pump into its latched position, fluid must be prevented from passing through the inner tube assembly. Also, once the inner tube assembly reaches its latched position, it is a requirement that fluid be allowed to pass through the inner tube assembly as part of the drilling process to flush away cuttings.
In order to achieve both of these functions, a release valve is installed within the inner tube assembly to control fluid movement during the various phases of pumping in and drilling.
Accordingly, as the inner tube assembly is being pumped into position, the release valve is in a first position that closes fluid flow ports that would otherwise allow the fluid to flow past the inner tube assembly. Upon the inner tube assembly reaching the core barrel and drilling position, the release valve will be maintained in a closed position which then results in a pressure spike which gives the operator at the surface an indication that the inner tube assembly has reached its latched position.
This increase in pressure may or may not be maintained up until the operator opens a valve to release the pressure. The release valve is arranged so that is will move to a second position itself or upon release of the pump in fluid pressure where the fluid flow ports within the inner tube assembly are open so that the latched inner tube assembly is ready for drilling. In this position, the fluid flow ports are now open and the drilling fluid is able to pass through the inner tube assembly to perform normal drilling operations.
However, prior to commencement of drilling and pumping of the drilling fluid through the drill tube, fluid within the drill tube and the drilled hole is free to drain away from the hole. This is particularly the case in relation to a dry hole where fluid is able to naturally drain within the rock strata or in relation to holes which are horizontal or inclined upwardly.
Clearly, this is undesirable as the drilling operation is delayed while additional fluid is being pumped into the drill string in order to commence the drilling operation. It also requires a skilled operator to know when fluid is present at the drill bit so that the drilling is not commenced without the necessary lubrication, cooling and debris clearance that the drilling fluid provides. If the drill bit is run without the drilling fluid being present, then damage can occur to the drill bit.
It is against this background and the problems referred to above which has resulted in the development of the present invention.
Certain objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein by way of illustration and example, an embodiment of the present invention is disclosed.